Conventionally, when an excimer laser apparatus employing halogen gas is operated, the halogen gas is consumed, during the process of operation, by evaporation of the electrode materials and chemical reaction with the constituent material of the laser chamber. Conventionally therefore, control was performed as follows in order to compensate for the lowering of laser output produced by consumption of halogen gas.
Specifically, the laser output is obtained by passing through a discharge space electrical energy for laser excitation that was accumulated on a capacitor, the discharge being effected in laser medium gas; if the charging voltage of this capacitor is raised, laser output is increased. Conventionally therefore laser output was stabilized by detecting the laser output and controlling the value of the charging voltage in accordance with the results of this detection. Such control is called "power lock control" and this charging voltage will hereinbelow be called the "power lock voltage".
However, even with such control, if operation is continued for a long time, the efficiency of oscillation is lowered by consumption of halogen gas, with the result that the prescribed output cannot be maintained unless the charging voltage (power lock voltage) is progressively raised.
Conventionally therefore arrangements were made to attempt to cope with this consumption of halogen gas by arranging for supplementation with a fixed quantity of halogen gas when the charging voltage increased above some prescribed voltage.
Such a method of halogen gas supplementation according to the prior art will be described with reference to FIG. 29 to FIG. 31.
In more detail, FIG. 29 shows structural parts pertaining to gas supplementation of a typical fluorine-based excimer laser apparatus; in this case, there are provided, as a gas feed cylinder, a cylinder 41 that is charged with a halogen gas (F2, HCl etc) diluted with a buffer gas (Ne or He etc), a cylinder 42 charged with a diluent gas such as Kr, and a cylinder 43 charged with a buffer gas such as Ne or He; when effecting gas feed on start-up, gas feed to laser chamber 47 is effected by open/shut control of on/off valves 44, 45, 46, and, when supplementing the halogen gas after operation, gas feed is effected through on/off valves 48, 49 and "subtank" 50.
In more detail, when introducing new gas into laser chamber 47 before start-up, first of all, the old gas in laser chamber 47 was discharged by means of on/off valve 51 and vacuum pump 52.
Next, Kr gas is introduced into laser chamber 47 to a pressure of 40 torr from cylinder 42 through on/off valve 45; next, F2 gas diluted by Ne gas is introduced to a pressure of 80 torr from cylinder 41 through on/off valve 44; finally Ne gas is introduced from cylinder 43 through on/off valve 46 to make the overall pressure in laser chamber 47 2500 torr.
By such gas feed control, the gas composition within laser chamber 47 in this apparatus becomes F2:Kr: Ne=4:40:2456 (torr), i.e. F2:Kr:Ne=0.16:1.60:98.24 (%) in terms of concentration ratios.
Thus, when laser chamber 47 is charged with new gas, gas supplementation control is performed by the procedure shown in the flow chart of FIG. 30 during subsequent laser operation.
First of all, before operating the excimer laser apparatus, the target laser output Ec, optimum control charging voltage range Vm (Vmin to Vmax), the increase/decrease charging voltage .DELTA.V when control is exercised once, and the one-time supplementation gas amount .DELTA.G are set beforehand (step 500).
When operation is then started, controller 55 gets the laser output E detected by laser output monitor 53 and the charging voltage V detected by charging voltage detector 54 (step 510). Controller 55 compares detected laser output E with the target laser output Ec (step 520); if E&lt;Ec, it raises the detected charging voltage V by the minute voltage .DELTA.V and makes this the instruction charging voltage Va (step 530); if E=Ec, it leaves the detected charging voltage V unaltered and takes this as instruction charging voltage Va (step 540); if E&gt;Ec, it lowers the detected charging voltage V by the minute voltage .DELTA.V and takes this as the instruction charging voltage Va (step 550).
Furthermore, controller 55 compares instruction charging voltage Va with the maximum value Vmax of the maximum control charging voltage range Vm (step 560); if Va&lt;Vmax, it returns again to step 510, to perform control of instruction voltage Va. However, if Va&gt;Vmax, supplementation of F2 gas containing Ne gas from cylinder 41 in a prescribed amount .DELTA.G is effected into laser chamber 47 (step 570), and some of the gas is discharged (step 580) so as to maintain the prescribed overall pressure of the gas in laser chamber 47.
FIGS. 31(a) to 31(d) show respectively time charts in respect of laser output E, instruction charging voltage Va, halogen gas (F2) concentration, and diluent gas (Kr) concentration resulting from the above control; the laser shot number is taken along the time axis. In FIG. 31(b), the time points t1 to t6 at which the charging voltage instruction Va to the capacitor suddenly drops correspond to the times when supplementation of halogen gas is effected.
However, with the prior art technique, gas cylinder 41 that is used for halogen gas supplementation does not contain a diluent gas constituent. Furthermore, with this prior art technique, every time gas supplementation occurs, control is exercised such as to maintain the overall pressure constant (step 580 of FIG. 30) by discharging some of the gas in laser chamber 47, so, every time gas supplementation is performed, the amount of diluent gas (Kr) is gradually decreased as shown in FIG. 31(d): a simple calculation shows that 20% of the Kr gas is eliminated by ten gas supplementations. In other words, this means that, as shown in FIG. 31(c), as gas supplementation of the halogen gas (F2) goes on, there will gradually be over-supplementation. Also, even if the one-time supplementation amount .DELTA.G is set sufficiently small so that over-supplementation does not occur, as shown in FIG. 31(c), the supplementation intervals then gradually decrease with the result that, in the end, the gas balance is destroyed. That is, with the prior art technique described above, every time supplementation of halogen gas is effected, the optimum compositional balance of the mixed gas in laser chamber 47 is lost, with the result that it becomes impossible to maintain a fixed laser output, no matter how the charging voltage to the capacitor is controlled.
Furthermore, with the prior art, there was the problem that, since halogen gas was supplemented in fixed amount every time in response to a comparison of the charging voltage but irrespective of the optimum compositional balance of the gas, it was difficult to maintain the optimum compositional balance, and the gas balance could easily be lost by external disturbances.
Furthermore, with the prior art, regarding gas exchange, this was always performed with the same gas composition, considered as optimum. However, with use of the laser for a long period of time, impurities and/or dust accumulate in the chamber so that there is a progressive fall-off in laser output. For this reason, even though gas exchange is performed, the charging voltage needed to obtain a prescribed laser output gradually becomes higher.
Furthermore, although, in some examples of the prior art, control of feeding of halogen gas is performed, this merely consists of the type of control in which a fixed quantity of halogen gas is intermittently fed, so there was the problem that the laser output was lacking in stability.
Also, with the prior art, there was no automatic notification of the time for changing the gas or the time when maintenance was due, so the operator had to make a decision about these from the dirtiness of the window or condition of deterioration of the gas, or the life of the laser chamber etc: this was difficult for an operator of limited experience.
Moreover, when laser oscillation is stopped, the gas in the laser chamber continues natural deterioration with lapsed time. However, in the prior art, no measures were taken regarding laser stoppage, so when laser oscillation was commenced, there was the problem that laser output was unstable. This was particularly marked if the laser was stopped for a long period.
With the foregoing in view, it is an object of the present invention to provide a method of gas supplementation for an excimer laser apparatus whereby loss of the optimum composition balance of the gas becomes unlikely even when gas supplementation is repeated many times; wherein the time point of gas supplementation and the amount of gas supplementation can be determined accurately; wherein stable laser output can be obtained even when the laser is stopped for a long a period; and wherein the operator is notified automatically and appropriately of the time for gas exchange and the time for maintenance.